A stereoscopic (3 dimensional) image can be created from two or more two dimensional images. The two dimensional images are taken from different positions, for instance by using different cameras at different positions, or by moving the same camera between positions.
For example, a camera can be placed onboard a moving object (termed platform), such as an aircraft, and successive images may be taken with at least 50% forward overlap, or more typically 56%-60% forward overlap. Forward overlap refers to overlap along (i.e. parallel) to the track of the platform. The stereoscopic view of the overlap corresponding area is produced based on those two successive images. The stereoscopic angle θs achieved for an approximately 60% forward overlap can be shown to beθs=0.4×(L/R)radians  (formula 1)                where L is the frame width (Frame width is the dimension of the footprint of the frame on the ground in the cross-line-of-sight direction which is the direction perpendicular to the line defined by the point on the ground beneath the camera and the point where the camera's line of sight points on the ground) and R is the imaging range (i.e. distance from the camera to the center of the imaged surface).These images may later be combined by means of a stereo viewing system to achieve depth perception.        
In order to provide wider coverage across, i.e. perpendicular or lateral, to the track of the moving platform, a still or video camera onboard the platform may “step” across the track, taking images which overlap with one another in the direction perpendicular to the track of the platform. Typically, in order to achieve a uniform or nearly uniform overlap between two given images adjacent along track from one another and two other images adjacent along track from one another and within the same two sequences as the two given images, the scanning is always performed from near to far (or alternatively from far to near). Therefore if a sequence of images perpendicular to the track was taken from near to far, the camera after finishing the sequence makes a large “step” so as to be able to take the next sequence of images also from near to far. Alternatively, a push-broom camera may sweep across the track, with the sweeping performed always near to far (or alternatively far to near).
U.S. Pat. No. 6,747,686 to Bennett describes an aerial reconnaissance camera and method for generating a first image of the terrain of interest with the camera pointing angle oriented or rotated about an azimuthal axis some angular amount (θ1) to a first, typically forward orientation, i.e. forward to a cross-line of flight direction. An image is obtained of the ground at this orientation. Then the camera is rotated about the azimuthal axis to new angular value (θ2), which will typically be aft of the cross line of flight direction. An image is generated at this value. This process of rotating the camera about the azimuthal axis and generating images in forward and aft orientations continues over and over. Eventually, as the aircraft flies past the terrain of interest, any given location will have been imaged from two different perspectives—forward and aft.
EP Patent Application 1 783 455 assigned to Rafael-Armament Development Authority Ltd. describes an airborne reconnaissance system comprising (1) Gimbals having at least two degrees of freedom; (2) At least one array of light sensors positioned on the gimbals, for being directed by the same within at least two degrees of freedom; (3) Map storage means for storing at least one Digital Elevation map of an area of interest, divided into portions; (4) Inertial Navigation System for real-time providing to a gimbals control unit navigation and orientation data of the aircraft with respect to a predefined global axes system; (5) Portion selection unit for selecting, one at a time, another area portion from the area of interest; and (6) servo means for directing the gimbals. The system uses data from the inertial navigation system and from the digital elevation map for real-time calculating direction to selected area portions, and for maintaining the direction during integration of light from the terrain, and for producing corresponding images of area portions.